1. Field of the Invention
The present invention relates to a method of molding a hollow molded article in which, in primary molding, semi-finished molded articles which make a pair are molded simultaneously so as to have joint faces by using a fixed die and a movable die capable of closing a molding device and, in secondary molding, the movable die with one semi-finished molded article remaining thereon is moved with respect to the fixed die to thereby cause the one semi-finished molded article to be superposed on the other semi-finished molded article so that their respective joint faces are opposed to each other, and the superposed joint faces are welded by closing the movable die with respect to the fixed die or by pressing the semi-finished molded articles in a molding device, thereby obtaining a hollow molded article from the semi-finished molded articles, as well as the hollow molded article, and an apparatus for manufacturing the same.
2. Description of the Related Art
A molding method using an injection molding machine is known as one method of manufacturing a synthetic resin-made hollow molded article which is obtained by complex joining, including a bent pipe, an intake manifold, a tank, and the like. This injection molding machine consists of a set of molding devices, as also shown in JP-A-62-87315 and JP-A-6-246781. A male die and a female die for forming one hollow molded article are provided in one molding device of the set of molding devices, while a female die and a male die for forming the other hollow molded article are provided in the other molding device. Accordingly, if, by using these molding devices, hollow products are formed as two split semi-finished hollow articles or split articles in primary molding, and their split surfaces are abutted against each other and a molten resin is injected and filled into the abutted joint space in secondary molding, the pair of semi-finished hollow articles are produced as one hollow product in which they are joined at the split surfaces. As a result, the hollow molded article can be manufactured by injection molding. According of this molding method using the injection molding machine, there are advantages in that it is possible to fabricate a hollow product which is completely sealed and fabricate a hollow product with a uniform thickness, and that it is possible to cope with complex shapes. However, since a molten resin for joining must be injected in secondary molding, the structure of the injecting machine becomes long or a long molding time is required, or the structure of the molding device becomes complex. Furthermore, if the secondary injection pressure is small, the joining force is weak, whereas if it is large to the contrary, there is a possibility of the molten resin leaking from the joint space to the interior of the hollow product.
Further, JP-A-6-305028 discloses a manufacturing method in which a tubular first member made of a thermoplastic resin and a similar tubular second member made of a thermoplastic resin are opposed to each other, an electrically conductive member is clamped by the opposing end faces, and the end faces are welded by subjecting the electrically conductive member to induction heating, thereby manufacturing a tubular article. In addition, the following method of manufacturing a hollow product is shown in JP-A-7-16945. Namely, in primary molding, first and second semi-finished hollow articles are molded simultaneously by using a slide die and a movable die so as to have joint portions. Then, in secondary molding, the slide die is slid upwardly with respect to the movable die, and one of the first and second semi-finished hollow articles molded in the primary molding is superposed on the other semi-finished hollow article such that their joint portions are opposed to each other. The inner surface sides of the joint portions of the superposed first and second semi-finished hollow articles are heated by a heater, or are heated by hot air so as to melt partially. Then, the joint portions are pressed and welded by a pressing member, thereby obtaining a hollow product from the first and second semi-finished hollow articles.
According to the invention disclosed in JP-A-6-305028, since it is unnecessary to inject a molding resin for joining when the end faces of the first and second members are joined, there is an advantage in that the injecting machine or the injection operation becomes simple. However, since foreign objects are embedded in the joint faces, there are also problems. For example, since the embedded electrically conductive member is made of a metal, there are cases where it exhibits a shielding effect with respect to radio waves, and cases where it generates heat, so that applications are sometimes limited. Furthermore, since the metallic electrically conductive member is interposed or embedded at the joint faces, i.e., since the joining is not that between synthetic resins, but that of different materials, i.e., a metal and a synthetic resin, the joining force can possibly be weak. In addition, although the JP-A-6-305028 does not especially show an apparatus used for carrying out the respective methods of molding the first and second members or for carrying out the method of joining the first and second members, it is supposed that the first and second members after being removed from the molding device are joined by using a special apparatus. Hence, these first and second members are likely to be affected by disturbances, and there are cases where their temperatures, shapes, and the like undergo change over time, possibly resulting in a decline in the precision of the molded product. In addition, it is estimated that the manufacturing process is made complex.
According to the invention described in JP-A-7-16945, since the inner sides of the joint portions are heated and welded in a state in the first and second semi-finished hollow articles are placed in the molding device, advantages are obtained in that the outer sides of the joint portions are not melted and deformed, and that only portions necessary for joining are melted, so that energy for melting can be small. Furthermore, since the secondary molding for joining the pair of semi-finished hollow articles is carried out by using the same molding device used in the primary molding for molding the pair of semi-finished hollow articles, there are advantages in that the apparatus for manufacturing a high-precision hollow product is inexpensive and that the hollow product can be manufactured at low cost. However, since the heater is embedded, the above-described problems are encountered. In addition, since the heater is in the form of wires, there is no guarantee that the end faces which are joined are uniformly melted. On the other hand, according to the method of joining by melting by hot air, since the hot air is large in compressibility and has a small heat capacity, it is estimated that it is technically difficult to supply hot air uniformly over the entire areas of the joint portions and to uniformly melt them. In addition, since the molten joint portions are adapted to be pressed and welded by a pressing member which is especially prepared, a higher cost of the manufacturing apparatus can result.